The invention relates generally to adaptive coding and modulation (ACM) and more particularly to ACM in a transmitter without the use of link parameters provided over a return link from a receiver in a communications system.
Adaptive coding and modulation (ACM), also known as link adaptation, is used in wireless communication systems to coordinate transmissions between transmitters and receivers. ACM involves adjusting various aspects of data transmission such as the type of modulation coding or bit error rates in accordance with information about the radio link, or channel. Channel state information (CSI) is commonly received via a return link from a receiver, forming a closed loop system. The receiver primarily provides information about how channel conditions are changing.
A representative prior art system 10 is shown in FIG. 1. At the beginning of a transmission, i.e. when a link is initially established, a transmitting terminal 12 chooses initial link parameters blindly, without a priori information, and begins transmission 14 to receiving terminal 16. Receiving terminal 16 receives transmission 14 and generates CSI, for example, an estimate of the signal to noise ratio (SNR) or bit error rate (BER) of the received signal. These are sent over return link 18 back to transmitting terminal 12. In response, transmitting terminal 12 adapts link parameters for subsequent transmissions based on the received CSI. Link metrics are sent at a rate that is commensurate with the rate of change for the channel of interest, with values typically ranging from seconds to tens of seconds.
Unfortunately, in many communications systems, providing a return link is challenging or impractical. Two such links are provided as examples. The first is an application in which a communications link is one directional, such as when the transmitting terminal is a telemetry or sensor link. In this application adding return link hardware, comprised of an additional receiver, low-noise amplifier, and diplexer to the transmitting terminal, solely for the purpose of receiving return link metrics, may be impractical due to the additional cost, size, weight, and power required. A second application where a return link is challenging or impractical is one in which a link is designed to minimize the probability of detecting the signal being transmitted. In this application, adding a secondary link in the opposite direction to relay CSI could conceivably double the chances of the signal being detected, which is an undesirable tradeoff.
Thus, a need exists for predictive link planning from a transmitting terminal and to a receiving terminal without a return link from the receiving terminal to the transmitting terminal. In addition, a need exists for ACM and predictive link planning in a system in which the geometry and link impairments between two or more communicating terminals can be predicted ahead of time.